The present invention relates to a method for desulfurizing gasoline, diesel fuel or like hydrocarbon fuel streams so as to render the fuel more suitable for use in a mobile vehicular fuel cell power plant assembly. More particularly, the desulfurizing method of this invention is operable to remove organic sulfur compounds found in gasoline to levels which will not poison the catalysts in the fuel processing section of the fuel cell power plant assembly. The method of this invention involves the use of a nickel reactant bed which has an extended useful life cycle due to the inclusion of oxygenates in the fuel stream in appropriate amounts.
Gasoline, diesel fuel, and like hydrocarbon fuels have not been useful as a process fuel source suitable for conversion to a hydrogen rich stream for small mobile fuel cell power plants due to the existence of relatively high levels of naturally-occurring complex organic sulfur compounds. Hydrogen generation in the presence of sulfur results in a poisoning effect on all of the catalysts used in the hydrogen generation system in a fuel cell power plant. Conventional fuel processing systems used with stationary fuel cell power plants include a thermal steam reformer, such as that described in U.S. Pat. No. 5,516,344. In such a fuel processing system, sulfur is removed by conventional hydrodesulfurization techniques which typically rely on a certain level of recycle as a source of hydrogen for the process. The recycle hydrogen combines with the organic sulfur compounds to form hydrogen sulfide within a catalytic bed. The hydrogen sulfide is then removed using a zinc oxide bed to form zinc sulfide. The general hydrodesulfurization process is disclosed in detail in U.S. Pat. No. 5,292,428. While this system is effective for use in large stationary applications, it does not readily lend itself to mobile transportation applications because of system size, cost and complexity. Additionally, the gas being treated must use process recycle in order to provide hydrogen in the gas stream, as noted above.
Other fuel processing systems, such as a conventional autothermal reformer, which use a higher operating temperature than conventional thermal steam reformers, can produce a hydrogen-rich gas in the presence of the foresaid complex organic sulfur compounds without prior desulfurization. When using an autothermal reformer to process raw fuels which contain complex organic sulfur compounds, the result is a loss of autothermal reformer catalyst effectiveness and the requirement of reformer temperatures that are 200xc2x0 F.-500xc2x0 F. higher than are required with a fuel having less than 0.05 ppm sulfur. Additionally, a decrease in useful catalyst life of the remainder of the fuel processing system occurs with the higher sulfur content fuels. The organic sulfur compounds are converted to hydrogen sulfide as part of the reforming process. The hydrogen sulfide can then be removed using a solid absorbent scrubber, such as an iron or zinc oxide bed to form iron or zinc sulfide. The aforesaid solid scrubber systems are limited, due to thermodynamic considerations, as to their ability to lower sulfur concentrations to non-catalyst degrading levels in the fuel processing components which are located downstream of the reformer, such as in the shift converter, or the like.
Alternatively, the hydrogen sulfide can be removed from the gas stream by passing the gas stream through a liquid scrubber, such as sodium hydroxide, potassium hydroxide, or amines. Liquid scrubbers are large and heavy, and are therefore useful principally only in stationary fuel cell power plants. From the aforesaid, it is apparent that current methods for dealing with the presence of complex organic sulfur compounds in a raw fuel stream for use in a fuel cell power plant require increasing fuel processing system complexity, volume and weight, and are therefore not suitable for use in mobile transportation systems.
An article published in connection with the 21st Annual Power Sources Conference proceedings of May 16-18, 1967, pages 21-26, entitled xe2x80x9cSulfur Removal for Hydrocarbon-Air Systemsxe2x80x9d, and authored by H. J. Setzer et al, relates to the use of fuel cell power plants for a wide variety of military applications. The article describes the use of high nickel content hydrogenation nickel reactant to remove sulfur from a military fuel called JP-4, which is a jet engine fuel, and is similar to kerosene, so as to render the fuel useful as a hydrogen source for a fuel cell power plant. The systems described in the article operate at relatively high temperatures in the range of 600xc2x0 F. to 700xc2x0 F. The article also indicates that the system tested was unable to desulfurize the raw fuel alone, without the addition of water or hydrogen, due to reactor carbon plugging The carbon plugging occurred because the tendency for carbon formation greatly increases in the temperature range between about 550xc2x0 F. and about 750xc2x0 F. A system operating in the 600xc2x0 F. to 700xc2x0 F. range would be very susceptible to carbon plugging, as was found to be the case in the system described in the article. The addition of either hydrogen or steam reduces the carbon formation tendency by supporting the formation of gaseous carbon compounds thereby limiting carbon deposits which cause the plugging problem.
It would be highly desirable from an environmental standpoint to be able to power electrically driven vehicles, such as an automobile, for example, by means of fuel cell-generated electricity; and to be able to use a fuel such as gasoline, diesel fuel, naphtha, lighter hydrocarbon fuels such as butane, propane, natural gas, or like fuel stocks, as the fuel consumed by the vehicular fuel cell power plant in the production of electricity. In order to provide such a vehicular power source, the amount of sulfur in the processed fuel gas would have to be reduced to and maintained at less than about 0.05 parts per million.
The desulfurized processed fuel stream can be used to power a fuel cell power plant in a mobile environment or as a fuel for an internal combustion engine. The fuel being processed can be gasoline or diesel fuel, or some other fuel which contains relatively high levels of organic sulfur compounds such as thiophenes, mercaptans, sulfides, disulfides, and the like. The fuel stream is passed through a nickel desulfurizer bed wherein essentially all of the sulfur in the organic sulfur compounds reacts with the nickel reactant and is converted to nickel sulfide leaving a desulfurized hydrocarbon fuel stream which continues through the remainder of the fuel processing system. Previously filed U.S. patent applications Ser. Nos. 09/104,254, filed Jun. 24, 1998; and Ser. No. 09/221,429, filed Dec. 28, 1998 describe systems for use in desulfurizing a gasoline or diesel fuel stream for use in a mobile fuel cell vehicular power plant; and in an internal combustion engine, respectively.
We have discovered that desulfurization of a gasoline or diesel fuel stream which uses a nickel catalytic adsorbant bed cannot be performed over a significantly extended period of time unless the fuel stream includes an oxygenate compound in appropriate proportions. Various oxygenates could suffice for the desulfurization process including MTBE, ethanol or other alcohols, ethers, or the like.
This invention relates to an improved method for processing a gasoline, diesel, or other hydrocarbon fuel stream over an extended period of time, which method is operable to remove substantially all of the sulfur present in the fuel stream. Examples of gaseous hydrocarbon fuels which can be desulfurized in accordance with this invention include methane, ethane, propane and butane.
Gasoline, for example, is a hydrocarbon mixture of paraffins, napthenes, olefins and aromatics, whose olefinic content is between 1% and 15%, and aromatics between 20% and 40%, with total sulfur in the range of about 20 ppm to about 1,000 ppm. The national average for the United States is 350 ppm sulfur. The legally mandated average for the State of California is 30 ppm sulfur. As used in this application, the phrase xe2x80x9cCalifornia Certified Gasolinexe2x80x9d refers to a gasoline which has between 30 and 40 ppm sulfur content, and which contains about 11% by volume MTBE at the present time. California Certified Gasoline is used by new car manufacturers to establish compliance with California emissions certification requirements.
We have discovered that the presence of oxygenates in the gasoline, like MTBE (methyl-tertiary-butyl ether, i.e., (CH3)3COCH3), or ethanol, for example, prevent rapid deactivation of the nickel catalytic adsorption of organic sulfur compounds from the fuel stream. Ethanol could be an appropriate solution to this problem since it is non-toxic, is not a carcinogen, and is relatively inexpensive and readily available in large supplies as a byproduct of the agriculture industry. Methanol, which would also extend the desulfurizer bed life, is not preferred since it is toxic; while MTBE is likewise not preferred since it is thought to be a carcinogenic compound, and may be banned in certain areas of the United States in the near future by new environmental regulations. Preferred oxygenates are non-toxic and non-carcinogenic oxygen donor compounds, such as ethanol or the like. When water is an oxygenate included ion the gasoline or diesel fuel mixture being desulfurized, the water content of the fuel mixture should be in the range of about 2% to about 5% by weight of the fuel mixture.
The effectiveness of a nickel adsorbent reactant to adsorb organic sulfur compounds from gasoline depends on the relative coverage of the active reactant sites by adsorption of all the various constituents of gasoline. In other words, the catalytic desulfurization process depends on the amount of competitive adsorption of the various constituents of gasoline. From the adsorption theory, it is known that the relative amount of adsorbate on an adsorbent surface depends primarily on the adsorption strength produced by attractive forces between the adsorbate and adsorbent molecules; secondarily on the concentration of the adsorbate in the gasoline, and temperature. Coverage of a reactant surface by an adsorbate increases with increasing attractive forces; higher fuel concentration; and lower temperatures. Relative to gasoline, Somoriai (Introduction to Surface Chemistry and Catalysis. pp, 60-74) provides some relevant information on the adsorption of hydrocarbons on transition metal surfaces, such as nickel. Saturated hydrocarbons only physically adsorb onto the nickel reactant surface at temperatures which are less than 100xc2x0 F., therefore paraffins, and most likely napthenes, won""t compete with sulfur compounds for adsorption sites on the nickel reactant at temperatures above 250xc2x0 F. and 300xc2x0 F.
On the other hand, unsaturated hydrocarbons, such as aromatics and olefins, adsorb largely irreversibly on transition metal surfaces even at room temperature. When an unsaturated hydrocarbon such as an aromatic or an olefin adsorbs on a transition metal surface, and the surface is heated, the adsorbed molecule rather than desorbing intact, decomposes to evolve hydrogen, leaving the surface covered by the partially dehydrogenated fragment, i.,e., tar or coke precursors. We have discovered that, at 350xc2x0 F., unsaturated hydrocarbons are nearly completely dehydrogenated, and the dehydrogenated tar fragments form multiple carbon atom-to-nickel reactant surface bonds. This explains why aromatics and olefins in gasoline, in the absence of oxygenated compounds in appropriate concentrations, will deactivate the nickel nickel reactant from adsorbing sulfur after a relatively short period of time.
In general, the adsorption strength of a compound depends on the dipole moment, or polarity, of the molecule. A higher dipole moment indicates that the compound is more polar and is more likely to adsorb on a reactant surface. Aromatics are an exception to this rule because their molecular structure includes a xcfx80 ring of electron forces that produces a cloud of induced attractive forces with adjacent surfaces. Based on the dipole moments of hydrocarbons, allowing for the xcfx80 ring in aromatics, the order of adsorption strength (highest to lowest) is: nitrogenated hydrocarbons greater than oxygenated hydrocarbons greater than aromatics greater than olefins greater than hydrocarbons containing sulfur greater than saturated hydrocarbons. Since the adsorption strength of the oxygenated hydrocarbons (such as ethanol, methanol, MTBE, or the like) is greater than that for aromatics and olefins, oxygenated hydrocarbons, or other oxygen donor compounds, if present in the gasoline or diesel fuel stream being desulfurized, will provide greater coverage of the nickel reactant sites than do the aromatics and olefins in the gasoline. Thus, the oxygenated hydrocarbons can reduce the adsorption of aromatics and olefins on the nickel reactant bed. Although saturated hydrocarbons (paraffins and cycloparaffins) would not be expected to be adsorbed on the desulfurization nickel reactant to a significant extent, oxygenated hydrocarbons will also prevent them from adsorbing onto the nickel reactant.
We have also discovered that the adsorbed oxygenated hydrocarbons do not inhibit the sulfur compounds from being adsorbed on the nickel reactant. The oxygenated hydrocarbons and the sulfur compounds are both quite polar and therefore they are miscible, which allows the sulfur compounds to dissolve into and diffuse through the adsorbed layer of oxygenated hydrocarbon to the active nickel metal reactant sites. Thus, the oxygenated hydrocarbons provide a xe2x80x9cshieldxe2x80x9d which inhibits the carbon-forming hydrocarbons from contacting the nickel reactant sites while allowing the sulfur compounds to contact and react with the active nickel metal reactant sites.
Further non-essential but enabling information relating to this invention will become readily apparent to one skilled in the art from the following detailed description of a preferred embodiment of the invention when taken in conjunction with the accompanying drawings in which: